Travelling wave tube coupler



Feb- 6, 1962 H. J. WOLKSTEIN ETAL 3,02

TRAVELLING WAVE TUBE COUPLER 2 Sheets-Sheet 1 Filed May 5, 1959 5 M; M N m m m W m m nw LIN B mm HR w w 3,020,444 TRAVELLING WAVE TUBE COUPLER Herbert J. Wolkstein, Newark, and Ralph E. Bridge,

South Bound Brook, N.J., assignors to Radio Corporation of America, a corporation of Delaware Filed May 5, 1959, Ser. No. 811,180 5 Claims. (Cl. SIS-3&3)

nited States arent to carry radio frequency energy. Because of the 'arrangement of the pre-formed embedded helical groove in the insulation of the coupler, the Wire in the coupler support is also completely embedded in the insulation. As a result, there is a very appreciable loss in the transfer of energy between the coupler and the medium with which the coupler is in energy transfer relation. In using such known coupler supports with travelling wave tubes, it has been found that each db of loss inserted into the system by the input coupler support results in a db of noise figure. Since travelling wave tubes are designed to carry a wide band of very high frequencies, each added db of noise figure resulting from loss in the coupler support reduces the eificiency and effectiveness of the travelling wave tube.

An object of the present invention is to enable the '4 efficient transfer of energy between the helix of a travelling wave tube and a helical coupler support external of coupler support which is relatively rigid and reduces to a minimum the loss encountered in transferring energy between the external coupler and an internal helix adapted to carry high frequency currents.

Still another object is to provide for use with a travelling wave tube a fluted external helical coupler in the form of a very thin hollow tube of insulation having on the interior surface thereof a plurality of spaced rows of ridges arranged to support an exposed wire wound between the rows.

In brief the invention comprises a low loss helical coupler support for use externally of the glass envelope of a travelling wave tube containing an internal helix. The coupler support is in the form of a thin hollow tube of insulating material, such as polytetrafluoroethylene known as Teflon, having on its interior surface a plurality of spaced rows of ridges arranged parallel to the longitudinal axis of the hollow tube of insulation. Each row of ridges is composed of a series of raised portions of insulation with an open valley or slot between adjacent ridges. These rows, preferably three in number, have their ridges so arranged that a coupler wire can be threaded in helical fashion through the open valleys or slots between the ridges, with the major portion of the wire exposed to the interior of the hollow tube of insulation. The coupler wire is thus supported by the sides of adjacent ridges in the respective rows. This arrangement permits a greater extension of the electric fields surrounding the Wire, thereby increasing the coupling coefiicient between the helix of the travelling wave tube and the surrounding helical coupler. The total effect is seen in the reduced insertion loss. Further, removal of a good portion of the insulation of the hollow tube, as compared to previously known coupler supports, significantly reduces the dielectric stored energy and enhances output. In addition to this, the reduction of dielectric "ice loading at high frequency reduces the wave velocity dispersion in the coupler. This is accomplished by the rows of ridges which preferentially load the low ends of the frequency band. In this manner, the coupler support provides a minimum of lossy material and a maximum of energy transfer between the wire mounted on the support and the helix of the travelling wave tube which it surrounds. The coupler of the invention thus meets the need of a low noise traveling wave tube Where a decrease in insertion loss is significant.

A more detailed description of the invention follows in conjunction with a drawing, wherein:

FIGURE 1 illustrates an embodiment of the low loss helical coupler support of the invention mounted upon the envelope of a traveling wave tube. Only a portion of the traveling wave tube is shown.

FIGURE 2 is a cross-section of FIGURE 1 taken along line 2-2; and

FIGURE 3 is a fragmentary view of a longitudinal section of the fluted hollow tube of insulation of the coupler support, showing the parallel rows of ridges of insulation material on the interior surface of the hollow tube. 1

FIGURES 4 and 5 are graphs relating insertion loss to distance for a known type of helical coupler support (FIGURE 4) mounted externally of a traveling wave tube as compared to the helical coupler support of the invention (FIGURE 5).

FIGURE 1 shows a portion of a traveling wave tube having an evacuated glass, envelope 10 of cylindrical cross-section surrounding an internal helical conductor 12 through the interior of which an electron beam is adapted to be projected. Since the traveling wave tube is well known in the art, and no claim is made herein to it per se, the details thereof including the source of electrons, the collector electrode, and the means for applying the magnetic fieid have not been shown in the interests of simplicity of illustration.

Surrounding the glass envelope 10 of the traveling wave tube is the low loss support for the helical coupler of the invention. The helical coupler support comprises a hollow thin tube of insulation 14, such as above-mentioned Teflon manufactured by Dupont, having on its interior surface a plurality of rows 16 of ridges 18 of insulation. The ridges 18 in each row are arranged in series, as shown, and are made of the same insulation material as the thin hollow tube 14. Adjacent ridges 18 in each row are separated by an open space or valley 20. The bottom of each open space or valley forms the smooth interior wall of the tube 14 of insulation. The three rows extend parallel to the longitudinal axis of insulatio-n tube 14 and are spaced apart, as shown in FIGURE 2. The rows of ridges are so arranged that the coupler wire 22 is threaded in helical fashion between the ridges of the three rows, as shown in FIGURE 1. The ridges 18 are smaller in height than the diameter of helical coupler 22 and constitute a minimum of lossy material to hold the wire 22 in position. Hence, the major portion of the wire 22 is exposed to the interior of tube 14.- for coupling with the internal helix 12 of the traveling wave tube, which the coupler Wire surrounds. It should be noted that the coupler wire 22 is wound in opposite sense to the inner helix of the travelling wave tube.

' In actual practice, in the manufacture of the coupler support of the invention, the insulation tube 14 is first threaded and then the flutes are made to provide the three spaced parallel rows 16, and then the rows are threaded to cut out the valleys or open spaces 20, thereby forming the ridges 18. Because the larger diameter wire is supported by the sides of the smaller ridges, there is an increase in the. coupling coeflicient between the two helices 22 and 12, with a consequent appreciable reduction in insertion loss. The removal of a considerable area of insulation around the wire 22, compared to the embedded type of prior art coupler supports, significantly reduces the energy stored in the dielectric and hence enhances output from or input into the coupler, depending on whether the coupler is used as an output or input coupler. The reduction in dielectric loading at the high frequencies to be passed by the traveling wave tube also reduces the wave velocity dispersion in the coupler. In practice, the ridge arrangement in the three parallel rows load the low end of the microwave frequency band to a greater extent than the high end of the band.

Surrounding the insulation tube 14 is a cylindrical metallic coupler support 24 to which is fastened a coaxial line 26 the inner conductor 28 of which is connected to a point at or near one end of the helical coupler wire 22.

The low loss coupler support of the invention may be used for both feeding input energy to the traveling wave tube and for abstracting energy therefrom. For this purpose, two couplers of the invention surround the envelope 10 as shown in FIGURE 1 and are suitably spaced apart along the length of the envelope 10.

FIGURE 4 is a graph relating insertion loss to distance between a pair of helical coupler supports of the type known in the art, mounted on a traveling wave tube. As indicated on the drawing, the loss per coupler varied from 3 db at 4800 megacycles to 4.8 db at 7000 megacycles.

FIGURE is another graph relating insertion loss to distance between a pair of helical coupler supports of the present invention, mounted on a traveling wave tube. The improvement, as summarized in the legend on this figure, shows that the loss per coupler varied from .9 db at 4000 megacycles to 2.5 db at 7000 megacycles. The reduction in loss by the use of the invention compared to the prior art coupler supports is highly significant, and now permits efiicient use of low noise traveling Wave tubes where a decrease in insertion loss is very important.

The slanted dotted line curves in FIGURES 4 and 5 are continuations or projections of the solid line curves. The solid line curves are the results of measurements. No measurements were made in the dotted line curve area because the two couplers could not be brought sufficiently close to each other on the envelope of the traveling wave tube to obtain measurements in this area.

In one embodiment of the invention successfully built and tested, the ridges of the fluted rows had a height of approximately 4 mils, While the diameter of the wire of the coupler coil held in position by the ridges was 11 mils. Obviously, the low loss coupler support of the invention is not limited to these dimensions. Similarly, insulation material other than Teflon can be used and of various thicknesses depending upon the performance characteristics desired.

What is claimed is:

1. Traveling wave tube apparatus comprising a traveling wave tube of the kind having a wave propagating helix inside the envelope, and a helical coupler coil external to the envelope and adapted to provide for the coupling of radio frequency energy to said helix, a support for said helical coupler coil comprising a hollow tube of insulation having on the interior surface thereof a plurality of spaced rows of ridges of insulation material, adjacent ridges in each row being spaced apart by an open valley, said rows being parallel to the longitudinal axis of said insulation tube, said helical coupler coil being held in place by the sides of the ridges in the different rows, the diameter of the wire of said helical coupler coil being greater than the height of said ridges and exposed to the envelope of the traveling wave tube which it surrounds.

2. The combination with a low loss, electromagnetic energy coupler support comprising a hollow tube of insulation having on the interior surface thereof a plurality of spaced rows of ridges of insulation material with said ridges each extending inwardly a given distance away from said interior surface, adjacent ridges in each row being spaced apart by an open valley, said rows being parallel to the longitudinal axis of said tube, of a wire wound helically around the interior of said tube in continuous contact with said interior surface and held in position by the sides of the ridges of said rows, the diameter of said wire being greater than the height of said ridges.

3. The combination with a low loss, radio frequency energy coupler support comprising a hollow tube of insulation having formed on the interior surface thereof a plurality of spaced rows of ridges with said ridges each extending inwardly a uniform distance above said interior surface, adjacent ridges in each row being spaced apart by an open valley, said rows being parallel to the longitudinal axis of said tube, of a wire wound helically around and continuously contacting the interior surface of said tube, said wire being held in position by the sides of the ridges of said rows, the diameter of said wire being greater than the height of said ridges.

4. In combination, a helix adapted to carry radio frequency energy and positioned within an evacuated nonelectrically conducting envelope, a coupler unit in energy transfer relation to said helix comprising a hollow tube of insulation having the same general cross-sectional shape as that of and surrounding said envelope, a metallie cylinder surrounding said tube of insulation, said tube having on the interior surface thereof three parallel rows of ridges of insulation material spaced apart, said ridges and tube being made of the same material, adjacent ridges in each row being spaced apart by an open valley, said rows being parallel to the longitudinal axis of said tube, a wire wound helically around and contacting the interior surface of said tube, said wire being held in position by the sides of the ridges of said rows, the diameter of said wire being greater than the height of said ridges, and a coaxial line having an inner conductor coupled to one end of said wire.

5. The combination with an electromagnetic wave carrying medium, of a pair of coupler units in energy transfer relation to said medium, said units being spaced apart along the length of said unit, each of said units comprising a hollow tube of insulation having on the interior surface thereof a plurality of spaced ridges of the same insulation material as said tube, adjacent ridges in each row being spaced apart by an open valley, said rows being parallel to the longitudinal axis of said tube, of a wire wound helically around and contacting the interior surface of said tube, said wire being held in position by the sides of the ridges of said rows, the diameter of said wire being greater than the height of said ridges.

References Cited in the file of this patent UNITED STATES PATENTS 2,531,972 Dohler et a1 Nov. 28, 1950 2,653,270 Kompfner Sept. 22, 1953 2,752,523 Goodall June 26, 1956 2,849,651 Robertson Aug. 26, 1958 2,894,168 Wing et a1 July 7, 1959 FOREIGN PATENTS 984,595 France Feb. 28, 1951 767,563 Great Britain Feb. 6, 1957 OTHER REFERENCES Magid et al.: Helix Coupling for Traveling Wave Tube, RCA Technical Notes No. 197. Received Aug. 18, 1958. 

